Selenium compound and its use as a brightener in a copper plating bath



United States Patent v s 480 524 SELENIUM coMPoUND AND ITS USE AS A BRIGHTENER IN A COPPER PLATING US. Cl. 20452 6 Claims ABSTRACT OF THE DISCLOSURE The compound of the formula and its use as a brightener in copper electroplating from cyanide baths.

This invention relates to a new composition of matter and the use of such composition in electroplating from copper cyanide baths, more particularly to a bath composition which is particularly adapted to producebright coatings of soft, ductile electroplates of copper.

Numerous attempts. to obtain such bright coatings have been made. For instance, it has been proposed to use alkali metal selenite as an addition agent to an electrolytic bath in relatively large amounts. Such baths had a number of disadvantages in that the selenites tended to break down with a resultant adverse effect on the bright plating range. During the electrolysis because of breakdown of the selenites, the anodes became blackened, forming insoluble compounds which were loosened from the anodes and tended to co-deposit with the plated metal, resulting in rough deposits which were commercially unuseable. Also in commercial operation, due to the resulting very narrow bright plating range, non-uniform appearance of the deposits resulted, and the deposits were in many cases insufiiciently bright so that buffing was necessary.

Another teaching of the prior art may be found in US. Patent 2,770,567 in which selenium compounds having a valence of .-2 are claimed as brighteners in alkaline copper plating. The use of selenium compound having a valence of 2 improved the brightness of the deposit .over a wider current density range but in comparison with the instant invention the operable current density range is much less desirable.

The present invention is intended and adapted to overcome the difficulties and disadvantages inherent in prior electroplating baths of the type described. It is among the objects of the present invention to provide a bath composition in which a relatively small amount of addition agents is introduced resulting in clearly increased brightness with the plated surface, with a wider more uniform plating range.

It is also among the objects of the present invention to provide a bath composition which results in a plating which is brilliant, soft and ductile and which may be readily buffed if desired to cause the coating to flow, as for example, on steel.

3,480,524 Patented Nov. 25, 1969 It is further among the objects of the present invention to provide a bath in which there is no breakdown even after long use.

The brightening additive of the instant invention is a selenium compound wherein the valence or coordination number of selenium is 4 and has the following structure.

C-OHa The selenium compound of the instant invention is the reaction product formed on reacting acetyl acetone and selenium dioxide. This reaction takes place on contacting the two reactants at a moderate temperature, for example, 70 C., approximate stoichiometric amount of the reactants are preferably employed, i.e., two moles of acetyl acetone per mole of selenium dioxide.

A typical preparation of this compound of the instant invention would be to charge a half liter three neck round bottom flask equipped with thermometer and reflux condensor in which is placed grams of acetyl acetone and 10 grams of selenium dioxide. The mixture is then heated while stirring to about 70 C. The reaction is completed in a very few minutes and the reaction product is cooled to room temperature and has the appearance of an orange to red liquid. This material can be used directly as a plating addition agent or it can be further purified by filtering to remove any trace quantities of metallic selenium and distilled in vacuo to remove excess acetyl acetone and traces of another component which is apparently formed by the reaction in trace quantities. The trace quantities of this other component is probably 2,3,4-pentanetrione and can be detected by gas chromatography. After this purification the residue is a thick red oil which on standing solidifies to a dark orange mass. After multiple recrystallizations from benzene the compound of the instant invention is a bright yellow crystalline solid having a melting point of 69- 70 C. Results of the elemental analysis, .molecular weight and infrared data confirmed the structure given above.

The compound of the instant invention is operable as a brightener in an alkaline copper plating bath at concentrations between about 0.005 and 0.5 gram per liter. The preferred concentration range for the compound of the instant invention in the copper plating bath is between 0.01 and 0.2 gram per liter. In this preferred range there is very little visible difference in deposit ap pearance throughout the range which is advantageous in that the exact concentration is not highly critical.

The use of the additive of the instant invention produces a brightening effect with all of the known alkaline copper plating baths. Such alkaline copper plating baths are typified by the plating baths given in the examples that follow. These alkaline copper baths operate at temperatures ranging from about F. to about F. and the current density can vary widely as can be discussed later in detail. In the following examples, all proportions are in grams per liter and selenium compound refers to the compound of the instant invention as discussed supra.

EXAMPLE I These copper plating tests were conducted in a Hull cell of 500 cc. capacity. Brass Hull cell panels (2 /2 in. x 4 in.) were used as the plating substrate and normal agitation was used during plating. Plating was carried out at 3 amps. for 5 minute periods and the bath tempera- 'ture was maintained at a temperature between 150-160 F. The bath composition was as follows:

Copper cyanide (CuCN) 75 Free potassium cyanide (KCN) 17 Potassium hydroxide (KOH) 25 Potassium carbonate (K CO 50 Selenium compound .01-.1

The resulting electroplate was semibright in the current density range 5 a.s.f., fully bright from 5120 a.s.f. and dull from 120-140 a.s.f.

EXAMPLE II Procedure of Example I was followed and the only variation in composition was that 0.01 gram per liter dithioammelide was added to the bath. A fully bright deposit was obtained throughout the current density range of from 0-120 a.s.f.

EXAMPLE III Following the procedure of Example I, a bath of the following composition was tested.

Copper cyanide (CuCN) 75 Free sodium cyanide (NaCN) 12 Sodium carbonate (Na CO 35 Sodium hydroxide (NaOH) 20 Selenium compound 0.01-0.1

The resulting copper electroplate was semibright in the current density range of 0-3 a.s.f., fully bright from 3-70 a.s.f. and dull from 70140 a.s.f.

EXAMPLE IV The procedure using the bath of Example III was repeated with the further addition of 0.01 gram per liter dithioammelide. This resulted in a fully bright deposit of copper from 0-70 a.s.f.

As will be noted from Examples II and IV, the addition of a small amount of dithioammelide improves the brightness in the extremely low current density areas. The use of dithioammelide and its equivalents are disclosed in detail in US. Patent No. 2,862,861 and describe the operable dithioammelide or its equivalents as a thio substituted six member heterocyclic ring compound wherein the members of the ring com-prise 13 nitrogen atoms and the balance carbon atoms, each nitrogen atom forming a part of a separate azomethine group and being connected to two carbon atoms, at least one carbon atom of one azomethine group carrying a substituent of the class consisting of thiolmercaptide and alkylthiol. Typical examples falling under this description are 2,4,6-trimercapto triazine, dithioammelide, thioammeline, 4,6-diamino-2- mercapto pyrimidine, 4,6 diamino 2-methylmercapto pyrimidine, 2,4-dimercapto pyrimidine, 4,amino-6-hydroxy-2-mercapto pyrimidine, 6,amino 4 hydroxy-2- methyl mercapto pyrimidine, 4,hydroxy 2-mercapto-6- methyl pyrimidine, and 2, mercapto pyridine. All of these compounds are useful in conjunction with the compound of the present invention to improve the brightness of copper electroplate in the extremely low current density areas. However, care should be taken to hold the concentration of these heterocyclic compounds below 0.05 gram per liter if a dull electroplate is to be avoided in the medium current density areas. However, if all plating is done in the low current density range up to 10 grams per liter of these compounds can be used to affect a fully bright plate in the extremely low current density areas. Preferably, however, the concentration of dithioammelide is held between 0.001 and 0.05 gram per liter.

A broad range of copper cyanide concentration may be employed with the brightening agents of the invention, and, in this respect, the copper cyanide concentration may range from about 45 to about 225 grams per liter. When no agitation is employed, best results are obtained when the concentration is from about 100 grams per liter to about 120 grams per liter. When the concentration falls below about 100 grams per liter it has been found that the brightness is adversely affected, and that below a concentration of about grams per liter only dull deposits result in the absence of agitation. In general, an increase in the copper cyanide concentration above about 120 grams per liter does not appreciably affect the brightness of the deposit obtained in nonagitated solutions, and concentrations as high as about 225 grams per liter may be employed. On the other hand, when the plating solution is agitated, it has been found that the preferred copper cyanide concentration should be materially reduced for the obtainment of optimum results. Thus, when vigorous agitation is employed, a copper cyanide concentration ranging generally from about 55 to about 85 grams per liter has been found best. With vigorous agitation as heretofore described it has been found, generally, that the brightness of the copper deposit is adversely affected when the concentration of copper cyanide falls outside a range which extends from about 45 to about grams Per liter. When the solution is agitated, therefore, it will be found that the brightness contributed by the disclosed compound is seriously affected unless the copper cyanide concentration is adjusted to compensate for the degree of agitation, or vice versa.

As is well known to those skilled in the art of copper cyanide plating, a complex potassium cuprous cyanide compound is formed between the potassium cyanide and copper cyanide. The actual formula of the complex varies according to the temperature conditions of the solution among other things. In this regard, the potassium cyanide concentration excluding the free potassium cyanide generally amounts to about 1.46 times the amount of copper cyanide employed. Thus, for a bath containing copper cyanide in amounts, between about 45 and 225 grams of copper cyanide per liter should contain potassium cyanide excluding the free cyanide, of from about 65 to about 325 grams per liter to form the complex. Similarly, for a solution having amounts of copper cyanide ranging between about and grams of copper cyanide per liter the potassium cyanide concentration, excluding the free cyanide, should be between about and grams of potassium cyanide per liter. The potassium cyanide concentration is thus based upon a potassium copper cyanide complex having the approximate formula K Cu(CN) It is necessary in copper cyanide solutions to have sufficient cyanide present to form a complex, otherwise, unstable conditions result. In this regard, there must be an excess of free cyanide such as sodium or potassium cyanide to insure that the complex is formed.

It has been found that generally greater quantities of free cyanide must be present when the solution is agitated than when the solution is not agitated. For example, when the solution is not agitated, it has been found that when the concentration of the free cyanide falls below about 2.5 grams of free potassium cyanide per liter, a dull deposit results. A similar dullness is detected above a free cyanide concentration of about 25 grams per liter. The best results in nonagitated solutions are obtained when the free cyanide concentration ranges from about 4 grams per liter to about 10 grams per liter. On the other hand, when vigorous agitation is employed as described heretofore, it has been found that best results are obtained when the free cyanide concentration ranges generally from about 9 to about 12 grams per liter. With vigorous agitation excessive anode polarization occurs, and a thick black coating is formed on the anode surface when the free cyanide concentration falls below about 6 grams per liter. When the concentration of free cyanide exceeds about 13 grams per liter, the cathode efiiciency is decreased appreciably. On the other hand, where no agitation is employed, it is found that the free cyanide concentration may fall as low as about 2.5 grams per liter, below which, however, excessive anode polarization occurs, and a thick black coating is formed on the anode surface. Usually the free cyanide concentration in excess of about 17 or 18 grams per liter is not recommended since the cathode efliciency decreases above this concentration due to excessive gassing, and only dull deposits are obtained above about 25 grams per liter of free cyanide. It will be apparent because of the necessity for free cyanide that the maximum limiting quantity of potassium cyanide is about 350- grams per liter when the copper cyanide concentration is about 225 grams per liter. The 350 grams per liter of potassium cyanide being the sum of the potassium cyanide associated with the copper cyanide and the free cyanide.

In preparing the basic solution, copper cyanide is preferably added to an aqueous solution of potassium or sodium cyanide in the desired amounts, and potassium or sodium hydroxide thereafter added to obtain the desired operating pH range. In general, a wide range of pH may be tolerated in the solution with optimum results being obtained in the pH range between about 11.5 and 12.5 in the case where there is no agitation, and in a pH range between about 12 and 13.5 in the case where there is vigorous agitation. Good results may be obtained in either case, however, at pHs ranging from about 9 to 14.

To buffer the solution against changes in pH, potassium citrate in amounts generally ranging from about 35 to about 75 grams per liter has been found suitable. Best results have been obtained when the potassium citrate is employed in amounts ranging from about 45 grams per liter to about 65 grams per liter. It will be apparent that it is not essential that potassium citrate be employed as a buffer or, in fact, that any butter be utilized in the plating solution. Other butters such as Rochelle salts may be employed also, and, in this regard, the Rochelle salts may be employed generally in amounts ranging from about 5 to about 55 grams per liter. Usually the Rochelle salts have been found to be more effective in the agitated solutions than in the nonagitated solution.

With respect to the temperature of the plating solution during the plating process optimum results in nonagitated solutions may be obtained over a somewhat wider temperature range the in the case where vigorous agitation is employed. For example, temperatures ranging from about 140 F. to about 160 F. may be employed without agitation of the solution, whereas, a temperature from about 150 F. to about 160 F. is preferably employed when there is vigorous agitation. Again, it will be apparent that the ranges set forth herein with respect to an agitated and nonagitated solution are principally illustrative of the invention, and that with varying degrees of the agitation the concentrations and operating conditions will vary so far as their optimum is concerned. Between the extreme case of agitation illustrated, and the other extreme where there is relatively no agitation, solution temperatures of from about 130 F. to about 185 F. may be employed. Although temperatures as low as 130 F. have been successfully employed, they are not strongly recommended since the brightness of the plate obtained tends to diminish if the temperature of the plating solution falls below about 140 F. in the case where agitation is not employed. On the other hand, when the temperatures in excess of about 160 F. are employed Without agitation slightly higher current densities should be utilized. The bright plating current density range tends to increase somewhat when higher temperatures are employed. In low current density areas the brightness usually diminishes when the temperature is increased much above 160 F. without a compensating increase in the average current density employed. It is pointed out, however, that temperatures as high as about 185 F. have been employed with success utilizing the brightening agent described herein.

Broadly, current densities up to about 120 amps, per

square foot of cathode surface area may be utilized according to the invention. Interrupted current appears to permit a wider and somewhat higher range of current densities and is usually preferred since it aids in eliminating polarization, and further minimizes film deposits on the anode. Suitable cycles for the use of interrupted current may have an on time of up to about seconds and an off time of from about 5 to about 50% of the on time. With a continuous current, optimum results in the form of maximum brightness have been obtained in nonagitated solutions when the current density ranges from about 10 to about 35 amps. per square foot of cathode surface area, whereas, with an interrupted current, one may use from about 10 to about 40 amps. per square foot without agitation. On the other hand, when vigorous agitation and a continuous current are employed, it has been found that a preferred range of current densities from about 10 to 20 amps. per square foot is best, whereas, with an interrupted current, the range is broadened out and may be raised to from about 10 to 60 amps. per square foot.

1. An electrolytic bath for the plating of bright copper comprising an aqueous alkaline solution of a cyanide of copper and an amount of a brightening additive suflicient to exert a brightening elfect on the deposit, said brightening additive having the formula 2. An electrolytic bath for the plating of bright copper as stated in claim 1 wherein said brightening additive is present in concentrations between 0.005 and 0.5 gram per liter.

3. An electrolytic bath for the plating of bright copper as stated in claim 2 wherein said brightening additive is present in concentrations between 0.01 and 0.2 gram per liter.

4. An electrolytic bath for the plating of bright copper comprising an aqueous alkaline solution of a cyanide of copper and an amount of a brightening additive sufficient to exert a brightening effect on the deposit, said brightening additive having the formula and a secondary brightener consisting of a thio-substituted six-member heterocyclic ring compound wherein the members of the ring comprise from one to three nitrogen atoms and the balance carbon atoms, each nitrogen atom forming a part of a separate azomethine group and being connected to two carbon atoms, at least one carbon atom of one azomethine group carrying a substituent of the class consisting of thiol, mercaptide and alkylthiol, said alkylthiol having from one to three carbon atoms, said thio-substituted compound being disolved in said bath in an amount from about 0.001 gram per liter to about 0.05 gram per liter.

5. An electrolytic bath for the plating of bright copper as stated in claim 4 wherein said brightening additive is present in concentrations between 0.005 to 0.5 gram per liter.

=6. An electrolytic bath for the plating of bright copper as stated in claim 4 wherein said brightening additive is present in concentration between 0.01 and 0.2 gram per liter.

References Cited UNITED STATES PATENTS DANIEL E. WYMAN, Primary Examiner 10 C. -F. DEES, Assistant Examiner US. Cl. X.R. 

